1. Field of the Invention
The present invention is directed to methods and apparatuses for separating gases from fluids in a fuel cell system and using gas to drive the transport of fluids within a fuel cell, and more particularly, to methods and apparatuses for separating carbon dioxide gas from an effluent flow in a direct methanol fuel cell system and using it to move fluids within the fuel cell system.
2. Background of the Prior Art
In recent years, fuel cells, which react hydrogen (extracted from a hydrogen carbonaceous fuel) with air or oxygen have been the subject of intensified development because of their ability to generate electric power from fuel without combustion. Fuel cell systems provide benefits of being able to produce electricity for as long as an appropriate fuel is supplied, low operating costs, and emitting only low levels of carbon dioxide, water and other byproducts of the reaction. Fuel cell systems may be divided into xe2x80x9creformer-basedxe2x80x9d fuel cell systems, which require some fuel processing to extract hydrogen from the fuel before introducing it into the fuel cell, and xe2x80x9cdirect oxidationxe2x80x9d fuel cell systems where the fuel is introduced into the fuel cell without processing. Adaptation of reformer-based fuel cell systems for use with small applications, such as personal electronic devices, has been difficult, due to the technical difficulties associated with reforming a carbonaceous fuel in a simple and cost-effective manner on a small scale. Hydrogen gas is not well suited to use in small fuel cell systems because its storage requires the use of high pressure vessels, metal adsorption matrices and/or cryogenic liquefaction of hydrogen gas to achieve useful energy densities.
A more convenient source of hydrogen is provided by carbonaceous liquid fuels such as methanol (and to a lesser extent ethanol, propanol, butane, and other hydrocarbons sources). Methanol has relatively weak chemical bonds that can be broken to free electrons to drive an electrical load. One way to convert the chemical energy in methanol to electricity is by using a direct methanol fuel cell (DMFC).
A direct methanol fuel cell system consists of a fuel cell, a fuel source, fluid and air management systems, a pump or mixing chamber, and other ancillary systems required for operation. The electricity-generating reactions occur within the fuel cell. The balance of plant components support the fuel cell by providing reactants to the cell, removing byproducts of the reactions, and otherwise facilitating the operation of the direct methanol fuel cell as it supplies power to the electrical load. It is common practice to connect multiple fuel cells in series, frequently in a xe2x80x9cstackxe2x80x9d configuration or in a planar system in order to meet the specifications of a given application. Although the present invention is described with reference to a direct methanol fuel cell system (DFMC system), it is understood, however, that the present invention may be suited for use with other direct oxidation fuel cells using other fuels.
In a DMFC system, methanol (typically in an aqueous solution) is supplied to the anode chamber of the fuel cell, which is separated from the cathode chamber of the fuel cell by a protonically-conductive (but electronically non-conductive) membrane (PCM). Catalysts which are present on the anode and cathode faces of the PCM, or which are otherwise present at the membrane surface) enable oxidation of the aqueous methanol fuel at the anode face of the PCM, separating the hydrogen protons and electrons from methanol and water molecules. Upon the closing of a circuit, the protons pass through the PCM, and the electrons are conducted through an electrical load, thus providing electrical power. The protons and electrons recombine with oxygen (preferably supplied by ambient air) on the cathode face of the PCM, forming water.
As a result of these reactions, DMFC systems produce only, water (H2O) and carbon dioxide (CO2) as byproducts of the reaction
The overall processes of a DMFC are as follows:
Anode: CH3OH+H2Oxe2x86x92CO2+6H++6exe2x88x92
Cathode: 3/2O2+6exe2x88x92+6H+xe2x86x923H2O
Overall: CH3OH+3/2O2xe2x86x92CO2+2H2O
Anodically generated CO2 is evolved in gaseous form and must be separated from the effluent solution and vented or captured for use within the system. If the CO2 is not vented, as is the case in a closed system, the CO2 pressure within the system will rise. If the pressure allowed to rise unchecked, the flow of the fuel mixture may be interrupted, or may cause a component or the system to fail.
As shown in FIG. 1, which illustrates one embodiment of a DMFC system, a direct methanol fuel cell system 2 includes a housing 4 defining an anode chamber 6 and a cathode chamber 8, a protonically conductive but electronically non-conductive membrane 10 (Protonically Conductive Membrane or PCM) and catalysts (not shown), a mixing chamber or pump 12, and a fuel source 14. Catalysts may be positioned anywhere within the anode and cathode chambers where they will come into contact with the fuel and water mixture, but are preferably is applied to both faces of the Protonically Conductive Membrane.
A gas separator device 16 is placed inline with a conduit that directs effluent from the anode chamber 6 to the mixing chamber. Similarly, water collector 18 is placed inline with a second conduit that directs effluent from the cathode chamber to the mixing chamber.
In gas separator 16, entering effluent contains both unused fuel solution (methanol and water), and CO2. The gas separator removes the CO2, and passes the unused fuel solution back to the mixing chamber using either passive management or a pump to induce said transport.
In water collector 18, water from entering cathodic effluent is collected and recirculated back to the pump or mixing chamber using either passive management, or a pump to induce said recirculation.
FIG. 2 illustrates another prior art fuel system, in which a gas permeable membrane is integrated into a chamber of the fuel cell. Accordingly, as CO2 builds up in the system, it is separated from liquid effluent by passing through the membrane.
In order to commercialize DMFC systems for use in small electronic devices, it is advantageous to remove as much CO2 as is possible from the anodic effluent solution in a simple, compact and cost effective manner. It is of further benefit to capture and utilize the carbon dioxide for use in the DMFC system to minimize parasitic power losses associated with actively managing and driving the fluidic components of the fuel cell system.
The present invention provides unique methods and apparatuses for transport of fluids in a DMFC system. In the present invention, gas from the effluent flow is allowed to collect on a coalescing surface. Upon reaching a predetermined volume, the product gas of the reaction shears a portion of liquid from the effluent. The released volume of the product gas transports the sheared liquid volume to a predetermined location where the gas is vented to the ambient environment, or directed to perform work within the DMFC. The sheared liquid volume may, for instance, be returned to the inlet flow of the DMFC, accordingly providing a self pumping action.
Accordingly, in one aspect of the present invention, a coalescing surface for inclusion into an element of a fuel cell system includes a vaulted wall having a domed shape.
In a second aspect of the present invention, a coalescing chamber for a fuel cell system includes a substantially closed container having an inlet for receiving effluent produced in a fuel cell and a coalescing surface comprising a wall having a domed shape.
In another aspect of the present invention, a fuel cell system includes a housing defining an anode chamber and a cathode chamber and including a catalyst, a protonically conductive but electronically non-conductive membrane positioned between the anode chamber and the cathode chamber, a mixing pump, a fuel chamber in fluid communication with the mixing pump, a first conduit having a first end connected to the anode chamber and a second end connected to the mixing pump, where the first conduit directs a fuel-water solution from the mixing pump to the anode chamber. The system also includes a second conduit having a first end connected to the anode chamber and a second end connected to the mixing pump, the second conduit directs effluent from the anode chamber to the mixing pump. A coalescing surface provided in the system collects effluent gas from the effluent received from one of the anode chamber and the cathode chamber.
The above identified aspects may be used in a method for separating gas from effluent produced in a chamber of a fuel cell system and includes passing effluent produced in the fuel cell adjacent the coalescing surface, and collecting gas from the effluent adjacent the coalescing surface.